Salmon is much higher in natural oils than Haddock. These are relativly 'healthy' Omega-3 oils, but depending on your cats weight this could be a factor to consider. Also, speaking from experience, Salmon tends to receive more 'alterations' from humans than other fish (ie they can offen be dyed orange if their flesh is naturally pale, or injected with growth hormones whilst in fish farms). Wild salmon is suprisingly rare, and very expensive.
Haddock is a much cleaner fish, but the price of haddock varies a lot throughout the seasons. If you do decided to do an all fish diet, Id buy up supplies when the fish is cheap and freeze it.
Some ocean fish, such as tuna and swordfish, have recently been critisized for having high levels of mercury in them. How true this is, or whether it even effects cats, I do not know.
As for fish bones, from the size of fish you'd be buying at a fish mongers, id say absolutly not to let the cats eat them. They're much to big and sharp for a cat to chew on safely.